1. Field of the Invention
The present invention relates to a playback system for recorded information, and more specifically to a system for playing back information recorded on a recording medium such as a video disc, which has recording areas respectively for a lead-in code and for a lead-out code, at the beginning and the end of a recording area for program signals.
2. Description of Background Information
Video discs may generally be classified into two groups, that is, the CAV (Constant Angular Velocity) disc and the discs recorded by other of recording methods. The discs of the latter category include CLV (Constant Linear Velocity) discs on which the signal is recorded while the disc is rotated so that the linear velocity of the recording track is constant, and CAA (Constant Angular Acceralation) discs on which the signal is recorded while the disc is rotated at a constant angular acceleration, which is described in the Japanese Patent Application No. 56-196633.
The CAV discs differ from the other types of discs (the case of the CLV discs is exemplarily explained hereinafter) in the density of the information, especially the information recorded in the peripheral tracks of the disc, in the speed of rotation of the disc, and in the capability of a still picture mode which is performed by a jump scanning process. Therefore, it is necessary to detect the recording type of the disc, to execute an appropriate playback process.
A video disc has an area for a program, and lead-in code area and lead-out code area which are respectively provided in the head portion and the end portion of the program area. The program area is for a program to be recorded, and takes the form of a continuous single area. In the case of the CAV discs, a Philips code which starts from a number "1" at the beginning of this area and increases by one, and is designated as a "frame number" is recorded by insertion in the program area. Further, in the case of the CLV discs, a Philips code indicative of the CLV disc, and designated as a "CLV code" is recorded in addition to the frame number. The frame number, a time code, or the CLV code is utilized for the discrimination between the CAV recording system and the CLV recording system, that is, the discrimination between the type of recording of the disc.
The lead-in code area indicates the beginning portion of the program area, and tracks having a width over 1.5 millimeters along the radial direction of the disc before the program area are used for this lead-in code, and filled with the Philips code to be used as the lead-in code.
However, in the areas for the lead-in code and the lead-out code, the frame numbers and the time code to be used for the discrimination of the above explained recording types, are not inserted.
Generally, playback systems for video discs are designed in such a manner that the recording types can be detected only in the program area, and the searching of the information and the playback of still pictures can be started only after completion of the detection of the recording type.
In addition, the lead-out area is to indicate the termination of the program area and in which a Philips code designated as the read-out code is recorded for at least six hundred tracks after the end of the program area.
The drawback of the conventional technique is as follows. In the playback system of this type of recording disc, the position on the recording disc, from which the tracking of the pickup device is started, is adjusted by means of a tracking arrangement including a limit switch.
Therefore, due to the deviation of the position or the operation of the limit switch, there have been cases in which the playback of the lead-in portion before the playback of the program area is relatively long, more than ten seconds for instance. Since the discrimination of the type of disc is not possible in the lead-in portion, such a long period of playback of the lead-in portion before the playback of the program area is wasteful. Especially, if the playback of a special address of the program area or a portion of the program area after a predetermined length of time is required, the period before the start of the search of information may become intolerable.
If, on the other hand, the limit switch is adjusted so that the initial position of the slider is close to the program area, for avoiding the above problem, there will be a fear that the program area, if it is very short, is omitted.
Further, in the case of the CAV disc in which a picture stop code (code for the playback of still picture), is inserted, the still picture is reproduced in a manner that an information scanning position on the recording disc is repeatedly jumped to the inner side of tracks, one by one, after the detection of the picture stop code. For enabling the still picture playback mode, the playback system must have previously detected the type of the disc loaded, i.e., the CAV disc should have been discriminated. However, if the picture stop code is recorded directly after the beginning of the program area, in the first track for example, the system cannot discriminate the CAV disc in the event that the frame number is not detected correctly by means of the playback system. As the result, the playback of the still picture according to the picture stop code is neglected. This kind of malfunction may occur due to a temporal drop of information, and the playback system may become out of control if the disc playback system is operated in accordance with "software" which is produced on the premise that the picture stop operation will be performed properly.